I got DH to take my first completed blocks to the post office to make sure of the postage. They told me $.44 - $.61 for one block, so I've been putting $.61 stamps on them. I can't remember the exact amount offhand for two finished blocks (in the same envelope), but two $.61 stamps were enough to cover it ... at least according to them. Sometimes I even put three $.61 stamps, just in case.
If anyone ended up having to pay postage due despite the rates I was given, let me know and I'll reimburse you.
FWIW, I haven't received any that had postage due.